GRLevel3 velocity question

[Dave Gallaher]

Had a question come to me via a PM. In the GRLevel3 table on the right in the Product Details box are the Max/Min readings. These change with tilts, and do not seem to correspond to the color table images onscreen. So when looking at a couplet that shows 65kts inbound and 70kts outbound SRV onscreen, the Max can be +116kts and the Min -102kts. What are the different readings corresponding to, and what represents the true measurement of the couplet?

As I write this, we are under a high wind advisory in north AL, with up to 35 steady, gusts to 50. BV is showing colors delineating +60 and -70, while Product Details are Max +115 and Min -123.

Thanks.

[Chris Vagasky]

See the attached image.

That is from SL4, but it works the same as on GRL3.

You can be looking at a tight couplet, but the max/min values are for across the whole radar image. They don't necessarily correspond to the couplet or your area of interest.

For a 'true' measurement of the couplet you would need to do a pixel query on the couplet to determine which I/O the colors correspond to.

[Jason Harris]

Glad to see this question raised (I had PM'd Dave), so the next part is how do you do a "pixel query on the couplet" in GR3? Also I noticed at one point with one of the storms in TN (or maybe it was AL) that in BV the couplet was much clearer than in SRV. I had the impression one should rely on SRV instead of BV to find most likely area of rotation? Or should one use both?

[Bob Hartig]

Jason, to do a "pixel query" (first I've heard that term, but it sounds cool! :-), just zoom in on the couplet and then mouse over individual pixels in the area of interest. In the bar at the bottom of your screen, you'll see a display that shows inbound and outbound winds in knots. Inbounds will be expressed as negative values and outbounds as positive values.

Regarding BV versus SRV, SRV will give you the better picture of rotational intensity because it factors storm speed and direction into the radar product. I don't know how much others on this site resort to base velocity, but I use it sparingly, usually on a hunch, to gain a different perspective on an area in question if SRV isn't painting a clear picture for me. I'll also try a higher tilt and/or a different radar site.

Note that level 2 data will give you much better detail than level 3. It's nice to have GR2 as an option.

[Dave Gallaher]

Thank you, Chris and Bob, for clearing that up.

I agree with Bob regarding BV--it is occasionally helpful, but I don't rely on it.

[Paul Knightley]

BV is more useful, IMO, for looking at severe 'straight line' winds, and large areas of surging ouflow.

[Dave Gallaher]

BV is more useful, IMO, for looking at severe 'straight line' winds, and large areas of surging ouflow.

I've always assumed I was looking at RFD when I found a couple scans of fairly broad high winds south of a couplet on SVR--BV usually clarifies it.